With the recent rapid spread of portable electronic devices such as cellular phones, smartphones, electronic books, and portable game machines, secondary batteries for drive power supply have been increasingly required to be smaller and to have higher capacity. Nonaqueous secondary batteries typified by lithium secondary batteries, which have advantages such as high energy density and high capacity, have been widely used as secondary batteries used for portable electronic devices.
A lithium secondary battery, which is one of nonaqueous secondary batteries and widely used due to its high energy density, includes a positive electrode including an active material such as lithium cobalt oxide (LiCoO2) or lithium iron phosphate (LiFePO4), a negative electrode formed of a carbon material such as graphite which is capable of occlusion and release of lithium ions, a nonaqueous electrolyte in which an electrolyte formed of a lithium salt such as LiBF4 or LiPF6, and the like is dispersed in an organic solvent such as ethylene carbonate or diethyl carbonate, and the like. A lithium secondary battery are charged and discharged in such a way that lithium ions in the secondary battery are transferred between the positive electrode and the negative electrode through the nonaqueous electrolyte and intercalated into or deintercalated from the active materials of the positive electrode and the negative electrode.
Into the positive electrode or the negative electrode, a binding agent (also referred to as a binder) is mixed in order that active materials can be bound or an active material and a current collector can be bound. Since the binding agent is generally an organic high molecular compound such as polyvinylidene fluoride (PVDF) which has an insulating property, the electron conductivity of the binding agent is extremely low. Therefore, as the ratio of the mixed binding agent to the active material is increased, the amount of the active material in the electrode is relatively decreased, resulting in the lower discharge capacity of the secondary battery.
Hence, by mixture of a conductive additive such as acetylene black (AB) or a graphite particle, the electron conductivity between active materials or between an active material and a current collector can be improved. Thus, a positive electrode active material with high electron conductivity can be provided (see Patent Document 1).